The recently privatized Search for Extra-Terrestrial Intelligence (SETI), with its
strong amateur component, has resulted in the deployment of dozens (soon hundreds, and
eventually thousands) of small amateur radio telescopes worldwide. [1] Generally operating
in the 1.3 to 1.7 GHz range, these receiving systems have been loosely based upon the
best amateur practice for 1296 MHz EME ("moonbounce") communications. A typical
example of such a system, built by SETI League member Daniel Fox, KF9ET, is seen in
Figure 1 below.

Figure 1
First generation Project Argus system by SETI League member Daniel Fox, KF9ET

The Spectral Dimension

Amateur SETI stations have shown themselves sufficiently sensitive to detect the
strongest likely signals from the nearest stars. [2] However, a significant limitation of these
amateur radio telescopes is their narrow instantaneous spectral coverage. With digital
signal processing being done at audio frequencies in multi-media personal computers, the
receivers employed are restricted to bandwidths of not more than tens of kiloHertz (a 12.5
kHz instantaneous bandwidth being typical). But the frequency region of greatest interest
(the so-called Water Hole) extends from 1420 to 1660 MHz, a total spectrum of 240
MHz. Scanning each 12.5 kHz channel for just 24 hours in meridian-transit (drift-scan)
mode, one station could cover the entire Water Hole at a single declination in just under
53 years. I submit that it will prove difficult to recruit volunteers for a sustained search of
such scope.

The so-called "professional" SETI programs (of which the now defunct NASA
SETI office is a prime example) devoted much of their resources to the development of
Multi-Channel Spectrum Analyzers (MCSAs) allowing the simultaneous scanning of tens
of MHz of spectrum, to resolutions typically in the tens of Hertz. Simple dimensional
analysis reveals that such receivers must typically operate on perhaps a million
simultaneous channels.

Outstanding MCSAs have now been deployed by the University of California,
Berkeley (Project SERENDIP), Harvard University (Projects BETA and META),
Argentine Institute of Radio Astronomy (META II), Ohio State University (Big Ear), the
SETI Institute (Project Phoenix), and others. Yet such broad spectral performance has
been, and probably remains, beyond the amateur state of the art. The present project
contemplates changing all that.

The META Model

The SETI MCSA which is probably best known to the ham radio community is the
Mega-Channel Extraterrestrial Assay. META was developed by Harvard University's
Prof. Paul Horowitz, W1HFA, and went on the air in 1985. Although hardly an amateur
effort (the development of the META-1 receiver was funded by a $100,000 grant to the
Planetary Society from filmmaker Steven Speilberg), Dr. Horowitz' amateur radio
background makes this an attractive benchmark for future ham efforts. The receiver,
occupying several six-foot high equipment racks, scans over 8 million simultaneous
channels, each 50 milliHertz wide, for a total instantaneous bandwidth of 400 kHz. Paired
with its twin in Argentina, Project META conducted one of the most exhaustive searches
ever for microwave signals of possible intelligent extra-terrestrial origin. [3]

Just ten years after the first META receiver went on its air, its technological
grandchild, BETA, was activated. BETA's 250 million channel MCSA operates at 0.5 Hz
resolution. Essentially a supercomputer running at 40,000 MIPS, the ambitiously named
Billion Channel Extraterrestrial Assay scans the entire Water Hole in eight bands, each 40
MHz wide, at a rate of two seconds per band. Although BETA represents Paul Horowitz'
greatest technological triumph to date, we expect even better performance from him in the
years ahead, as computer technology continues to advance.

STAR-1 Unveiled

STAR-1, the SETI League's first Spectral and Temporal Analysis Receiver (see
Figure 2 below) is a work in progress. Far from completion and not really intended for
duplication when finally operational, it is a test-bed for developing low-cost MCSA
technology for use by the world's radio amateurs and amateur radio astronomers.

STAR-1's frequency coverage, and much of its basic design, were dictated by an
accident of electromagnetic interference (EMI). It has been noted that the prime radio
astronomy spectrum extends between roughly 1420 MHz (the precession frequency of
neutral hydrogen atoms in space) and 1660 MHz (one of the resonances of interstellar
hydroxyl ions). Smack in the middle of this prime real estate, at 1575 MHz, is an angry
swarm of navigation satellites. The US's Global Positioning System (GPS) and its Russian
counterpart, GLONAS, make the center of the Water Hole all but unusable for terrestrial
SETI. [4] (See Figure 3, below).

In view of the above interference, it was felt reasonable to develop a receiver with
two spectral response bands, one in the vicinity of the hydrogen line, and the other
encompassing the hydroxyl line, with a gap in between. Since the two frequencies are 240
MHz apart, they can be images of one another if heterodyne downconverted to a common
120 MHz IF. The required local oscillator frequency would then fall halfway between the
two signal bands, or roughly in the already spectrally polluted GPS band!

An added bonus of excluding the middle of the Water Hole from analysis is that we
can now develop a precise frequency reference, based upon reception of GPS navigation
signals. This possibility is explored in the Section which follows.

As seen in the block diagram, STAR-1's second conversion is done directly to
baseband. By utilizing image-recovery technology, it is possible to extract two 6 MHz
wide outputs for each of the two input channels, for a total of 24 MHz real-time frequency
coverage. Signal analysis is then done in four advanced Digital Signal Processors, with
ultimate resolution limited primarily by the performance of the DSP engines.

Tom Clark's Clock

If we can't receive SETI signals in the GPS band, is there a way to capitalize on
the interference? It turns out there is: we can use GPS as our primary time and frequency
reference.

There are number of interesting things which a SETI enthusiast can do with a high
precision time and frequency standard. We can use them to calibrate or control our
receiver frequency coverage, determine accurate local sidereal time (and hence right
ascension of our drift-scan radiotelescopes), and to provide a common time reference for
correlating long-baseline interferometry observations. In the STAR-1 MCSA, such a
reference would facilitate the design of frequency synthesized local oscillators.
Unfortunately, the technologies used in the past for accomplishing these goals, Cesium or
Rubidium frequency standards and Hydrogen masers, are priced beyond the reach of most
experimenters. Thanks to a ham (of course!) that is no longer the case.

Dr. Thomas A. Clark (W3IWI), the NASA scientist whose name is virtually
synonymous with very long baseline interferometry, has been grappling with the time and
frequency standard problem for his geodesy experiments. By deriving timing information
from the atomic clock-controlled Global Positioning Satellites, he is able to produce a
relatively low cost time and frequency standard with long term accuracy rivaling a
Hydrogen maser.

Tom Clark's Totally Accurate Clock, or TAC (the acronym happens to be his
initials) produces a 1 PPS output exhibiting 30 - 50 nSec precision and 30 nSec rms
accuracy, even with Selective Availability implemented in the GPS system. At 1-day
averaging, the unit produces a standard frequency accurate to one part in ten to the
twelfth. This is equivalent to Cesium or Rubidium standards, at a cost of about $800.
Better still, Tom has made hardware and software details available in the public domain.
And as a special bonus, Tom has written SHOWTIME.EXE, a nifty program for
displaying GMT time, sidereal time, and user location on a PC screen. The program,
which is driven from Tom's GPS-based clock, is especially useful for radio astronomy
applications, and is being distributed free of charge.

Those SETI enthusiasts interested in assembling a Totally Accurate Clock can
obtain full hardware and software details via anonymous FTP from the following
directory: ftp://aleph.gsfc.nasa.gov/GPS/totally.accurate.clock/. Note that GPS is upper
case. Also check the Tucson Amateur Packet Radio (TAPR) web site at
http://www.tapr.org for news concerning the availability of the TAC-2 in kit form.
As seen in Figure 2, the first and second local oscillator frequencies for STAR-1
are synthesized in a phase-locked loop derived from a precision 10 MHz temperature
compensated crystal oscillator (TCXO). The multiplication chain for the first LO is
derived from an existing no-tune transverter board from Down East Microwave.

Since all LOs derive from the same TCXO, accuracy and frequency stability are
limited by the performance of a single stage. W3IWI is currently working on an add-on
for the TAC-2 to facilitate synchronization of just such a TCXO to GPS time. While we
await availability of that product (probably through TAPR), the STAR-1 prototype is
employing a surplus PTS commercial frequency synthesizer for its local oscillators.

The Front Ends

The low noise amplifier (LNA) shown in Figure 2 is a room-temperature
pseudomorphic high electron mobility transistor (PHEMT) stage. System design
objectives call for a 50 Kelvin noise temperature, at 20 dB gain, with the LNA mounted
directly at the antenna feed. It remains to be seen whether we can achieve the desired
bandwidth from a single LNA (or for that matter, from a single antenna feed). It is
expected that the final configuration will employ separate feedhorn probes, and separate
LNAs, for the hydrogen and hydroxyl bands.

The amplifier and bandpass filter stages which follow the LNA were built by
modifying two existing Down East Microwave 1296 MHz no-tune boards (see Figure 4).
It was necessary to raise the resonant frequency of the hairpin bandpass filters by trimming
0.1 inch and 0.3 inch lengths, respectively, off each end of each filter pole, for the
hydrogen line and hydroxyl line frequencies. This modification was done with an approxo
knife (there's nothing exact about it!)

The resulting gain curves, using three stages of GaAs MMIC amplification
between and behind filter poles, are seen in Figures 5 and 6. A Mini-Circuits mixer and multiple-MMIC second IF amplifier are tacked on to the outputs of the two front end
strips, with the bulk of the system gain designed into these VHF stages. Since radio
telescopes operate best without automatic gain control (AGC), a step attenuator has been
designed into each IF strip to optimize system gain.

So far, there's nothing particularly exotic about the receiver we've described. The
RF portion of STAR-1 represents a logical application of current amateur practice to a
specific set of design objectives. What makes or breaks any MCSA design is its digital
signal processing (DSP) hardware and software. As it happens, it is in the DSP area that
the design of STAR-1 is most tentative. Did I remember to say that this is a work in
progress? In that spirit, much of the DSP area is also vapor-ware at this writing.

It happens that our IF, though 72 MHz wide, is centered roughly on the 2 meter
band. It seemed logical, then, to process the VHF IF down to baseband in an image-reject
(or phasing type) direct conversion receiver, similar to that which Rick Campbell, KK7B,
has described for 2 meter use. [5] However, whereas Rick did his image-reject conversion
in two Mini-Circuits mixers, a Toko power splitter, and discrete LO phase-shift networks,
we were fortunate to find all these components in a single package. Shown as a mixer
with two outputs in the block diagram (Figure 2), that package is a commercial quadrature
(or I-Q) demodulator.

The detector selected, Mini-Circuits MIQC-176D-111, sells for $55 in single
quantities. Though a moderately costly component, when presented with IF signals and
ample LO injection the I-Q demodulator selected produces two broadband (DC to 6
MHz) outputs exactly in phase quadrature, with a maximum phase imbalance of plus and
minus 3 degrees, and amplitude balance within 0.4 dB.

Since we desire to analyze not kHz, but rather of MHz of spectrum at a time, we
need to detect the VHF IF not down to audio, but rather to baseband signals which more
closely resemble video. What better device to follow the IQ demodulator than a standard
video amplifier module, with its DC to 6 MHz bandwidth? Both the I and Q channels are
amplified, then digitized for input to a dedicated DSP chip. Note that since we are
simultaneously processing both the hydrogen line and the hydroxyl line band, a total of
four 12-bit A/D converters, each sampling at 12 MSPS, as well as four separate DSP
engines, will be required. An ordinary personal computer is used not for signal analysis,
but rather to control the four DSP engines, and for data storage and display.

High performance commercial DSP chips are evolving at a dizzying rate. This is a
far cry from the days of NASA SETI, when custom DSP engines had to be designed and
manufactured (at taxpayer expense). When this project was started, we contemplated
employing perhaps four Graychip EV-4014 evaluation boards, which were priced at
$6750 each. Fortunately, before we had set soldering iron to circuit board, Texas
Instruments announced their new TMS320C6x, a 1600 MIPS dedicated DSP chip which
was said to be able to perform a 1024 point Fast Fourier Transform (FFT) in 70
microseconds, and run at speeds up to 200 MHz. Price was announced at $96 in 25,000
quantity, with a developer's kit available for just (just?) $2995. Since new market entries
occur almost weekly, we're still not sure what DSP chip we'll ultimately use. But at this
writing (June 1997), the 'C6x (see http://www.ti.com/sc/C6x) is the odds-on favorite.

Note that if we analyze 6 MHz of baseband with a 1024 point FFT, our bin
resolution will be on the order of 6 kHz. To achieve our goal of 10 Hz resolution would
seem to require either analyzing only one thousandth of the baseband spectrum at a time,
or running 1,000 parallel DSP engines, in each of the four output channels. At this point,
some combination of the two approaches is contemplated. We feel this problem will solve
itself as even more powerful DSP chips become available, and the cost of the existing ones
continues to decrease.

In KK7B's direct conversion receiver designs, much attention was paid to
developing phase-stable 90 degree audio phase shift networks to allow the upper or lower
sidebands to be extracted from a pair of mixers fed in phase quadrature. You'll notice no
such phase shift networks following the video amplifiers in Figure 2. This is because I
could come up with no reliable way to achieve exactly 90 degrees of phase shift over a DC
to 6 MHz bandwidth. A technique widely used in monopulse radar systems is to digitize
the two phase-quadrature components, and then apply a Hilbert transform [6] to them in
software.

Since we already intended to digitize our I and Q components, SETI League
president Richard Factor (WA2IKL) recommended we employ the Hilbert transform
approach in STAR-1. The result is simultaneous USB and LSB reception, which allows
us to cover downconverted hydrogen line and hydroxyl line components which extend
from DC to both 6 MHz above and 6 MHz below the second LO frequency. When the
second LO is programmed for 120 MHz, for example, the receiver is simultaneously
processing 1414 to 1426 MHz (a 12 MHz slice of spectrum centered on the hydrogen
line) and 1654 to 1666 MHz (a similar chunk centered on the hydroxyl line). By tuning
the second LO in 10 MHz steps, this instantaneous 24 MHz of bandwidth allows us to
tune a total of 144 MHz of prime SETI spectrum with slight overlap.

If the four square-law detectors shown in Figure 2 appear to have been added as
an afterthought, it's because they were. Their purpose is to allow STAR-1 to function as a
total-power receiver, for conventional broadband radio astronomy observations. (The
required amplifiers and integrators may be accomplished in either analog circuitry or
software, yet to be defined). For years, such projects as Suitcase SETI, Sentinel, and
SERENDIP (operated by Harvard University and the University of California, Berkeley)
have enabled SETI scientists to tap in to the outputs of the world's great research-grade
radio telescopes, in order to perform what has become known as parasitic SETI. It seems
only appropriate that a receiver specifically designed for advanced SETI should
incorporate the capability of parasitic radio astronomy. Turnabout is indeed fair play.

Progress To Date

As of June 1997, the basic STAR-1 topology is relatively well defined. The front
ends, IF strips, I-Q demodulator and video amplifier circuits have all been breadboarded
and tested. The multiplier chain for the first local oscillator is operational, but since the
110 MHz GPS-locked PLL has not yet been developed, it is being driven by a 110 MHz
crystal oscillator for test purposes. At present the second LO is a GPIB-programmable
synthesized laboratory signal generator. And the A to D converters and DSP engines, the
true heart of any MCSA, are still a distant dream. The SETI League welcomes the
assistance of any and all interested radio amateurs. To participate in an email discussion
group dealing with SETI system design, send an email to "Majordomo@sni.net" with the
message "subscribe seti" in the body.

Conclusion: What's In A Name?

While preliminary development of the STAR-1 receiver was underway, the
scientific community lost its most eloquent spokesman. The untimely death of Carl Sagan
shook the SETI community especially, since he had been intimately involved in the Search
for Extra-Terrestrial Intelligence since 1961. It is not generally known that it was Sagan
who coined the name Mega-channel Extra-Terrestrial Assay, and the acronym META, for
Prof. Paul Horowitz' landmark 1985 MCSA design. After consulting with Horowitz, with
officials of the Planetary Society (of which Dr. Sagan was president), and especially with
Sagan's widow Anne Druyan, The SETI League has decided to rename the STAR-1
receiver project "mini-META" in his honor, and to dedicate this effort to the memory of
Carl Sagan. The name reflects NASA administrator Dan Goldin's watchword for future
space missions in the present economic climate: "smaller, faster, cheaper." Amateurs may
never best Horowitz' and Sagan's efforts. But two out of three ain't bad.

[4] This is the reason that SETI scientists are seriously contemplating construction of a radio
observatory on the far side of the moon, for deployment early in the next century. See
http://www.setileague.org/press/pres9707.htm for further details.